USB: f81232: implement port enable/disable method

We put FCR/IER initial step to f81232_port_enable()/f81232_port_disable().
When port is open, it set MSR interrupt on. Otherwise set it off.

Signed-off-by: Peter Hung <hpeter+linux_kernel@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>